Question

Methyl red has the following structure: CN(C)c1ccc(N=Nc2ccccc2C(=O)O)cc1 It undergoes a color change from red to yellow as a solution gets more basic. Calculate an approximate \(\mathrm{pH}\) range for which methyl red is useful. What is the color change and the \(\mathrm{pH}\) at the color change when a weak acid is titrated with a strong base using methyl red as an indicator? What is the color change and the \(\mathrm{pH}\) at the color change when a weak base is titrated with a strong acid using methyl red as an indicator? For which of these two types of titrations is methyl red a possible indicator?

Short answer

Methyl red is useful when the pH range is approximately between 4.4 to 6.2, transitioning from red to yellow. When a weak acid is titrated with a strong base using methyl red, the color change occurs at the midpoint of the pH range, which is around 5.3. Similarly, when a weak base is titrated with a strong acid using methyl red, the color change occurs at a pH around 5.3, transitioning from yellow to red. Methyl red is most suitable as an indicator for weak acid-strong base titrations, especially for weak acids with \(\mathrm{p}K_\mathrm{a}\) values around 5, but not typically appropriate for weak base-strong acid titrations.

Step-by-step solution

Determine Methyl Red's pH Range

Methyl red undergoes a color change from red to yellow as a solution gets more basic. The approximate pH range for methyl red is typically from 4.4 to 6.2. In this range, the color of the solution will vary from red (pH 4.4) to yellow (pH 6.2).

Color Change and pH for Weak Acid with Strong Base Titration

When a weak acid is titrated with a strong base using methyl red as an indicator, the color change will occur at the pH where the concentrations of the weak acid and its conjugate base are equal (the endpoint of the reaction). This is the result of the solution transitioning from acidic to basic, with the methyl red indicator marking this transition. The color change will happen at the midpoint of the pH range for methyl red, which is approximately \(\frac{6.2 + 4.4}{2} = 5.3\). Thus, the color change occurs when the pH is around 5.3, transitioning from red to yellow.

Color Change and pH for Weak Base with Strong Acid Titration

When a weak base is titrated with a strong acid using methyl red as an indicator, the color change will occur at the pH where the concentrations of the weak base and its conjugate acid are equal (endpoint of the reaction). This occurs as the solution transitions from basic to acidic. Similar to Step 2, the color change occurs at the midpoint of the pH range for methyl red (5.3) and transitions from yellow to red.

Determine Suitable Titration Type for Methyl Red Indicator

Methyl red is considered a useful indicator when the pH at the endpoint corresponds to its pH range. For weak acid-strong base titrations, the endpoint pH is typically around 5.3, which falls within the range of 4.4 to 6.2; therefore, methyl red can be used as an indicator for weak acid-strong base titrations. However, it is not suitable for all weak acid-strong base titrations. Methyl red is best suited for weak acids with a \(\mathrm{p}K_\mathrm{a}\) close to 5. For weak base-strong acid titrations, the endpoint pH usually falls outside the range of 4.4 to 6.2. Hence, methyl red is typically not an appropriate indicator for weak base-strong acid titrations. Overall, methyl red is a possible indicator for weak acid-strong base titrations, especially for weak acids with \(\mathrm{p}K_\mathrm{a}\) values around 5.